Car surgery

A mechanic was removing the cylinder heads from the motor of a car when he spotted a famous heart surgeon in his shop who was standing off to the side, waiting for the service manager to come take a look at his car. The mechanic shouted across the garage, “Hello Doctor! Please come over here for a minute.” The famous surgeon, a bit surprised, walked over to the mechanic. The mechanic straightened up, wiped his hands on a rag and asked argumentatively, “So doctor, look at this. I also open hearts, take valves out, grind them, put in new parts, and when I finish this will be as good as new one. So how come you get the big bucks when you and I are doing basically the same work?” The doctor leaned over and whispered to the mechanic “Try doing with the engine running.”